This project will examine the underlying factor structure of impulsive behaviors, and investigate the genetic basis of impulsive behaviors in healthy human volunteers. Impulsive behaviors are strongly implicated in risk for drug abuse, and are thought to be determined in part by genetic factors. Here, the investigators will examine the two main components of impulsive behavior: Impulsive Choice (maladaptive decision-making) and Impulsive Action (i.e., behavioral inhibition) using standardized behavioral tasks. They will empirically derive constructs based on subjects' performance on the tasks, and then examine polymorphisms in genes thought to contribute to variations in the constructs. The investigators will focus on genes that affect function of the dopamine system. The project addresses an urgent need to define behaviorally the underlying components of impulsivity, and to identify genetic factors that influence variability. The findings will help us understand impulsive behaviors, which are key risk factors, or intermediate phenotypes, for drug use. First, we will phenotype healthy unrelated young adults (N=1,000) on carefully selected behavioral measures of Impulsive Choice and Impulsive Action, and identify the factors comprising these heterogeneous behaviors. Participants will be tested twice for maximum reliability, and we will use factor analysis to identify the underlying latent factors. Second, we will examine associations between the derived factors and genetic variation in dopamine and other selected genotypes, in three levels of analysis. Using a candidate gene approach, we will focus on polymorphisms in selected genes related specifically to dopamine function. We hypothesize that genotypes resulting in low dopamine function will be associated with higher impulsive behaviors. Using a pathway-based approach, we will investigate polymorphisms in a larger set of genes of interest based on the published literature. Finally, using a hypothesis-free approach, we will examine ~1 million polymorphisms that will survey all the genes in the genome. Thus, we will include both hypothesis testing and exploratory approaches to comprehensively examine the genetic basis of impulsivity. The project is significant because it will advance understanding of impulsive behavior and its genetic underpinnings, which has direct relevance to risk for substance abuse. The study is innovative because it combines rigorous behavioral analysis with multi-level genetic analysis.